WebRTC SFUs encrypt traffic differently to the individual users. Each outgoing packet needs to be encrypted with the receiver's negotiated keys, is my understanding. If it was a shared key, then presumably that key could be negotiated among the peers using public key exchange, unbeknownst to the server, and then all traffic be e2e encrypted, which it isn't. I have minimal understanding of what protocols Zoom uses out of the box, but if they don't support e2e encryption, it seems to me they need the keys on the servers for a reason, and the only legit reason would be to trans-crypt the packets.